The present invention relates to a transmission for four-wheel drive vehicles, and more particularly to an improvement thereto which permits a smooth switching-over between the two-wheel drive mode and four-wheel drive mode while the vehicle is running, and which further permits a circulation of lubricating oil or hydraulic fluid between the front and rear transmission sections.
In a conventional four-wheel drive automobile, a switching-over between the two-wheel drive mode and four-wheel drive mode is accomplished by means of the gear clutch, and it is therefore necessary to bring the vehicle to a standstill or rest before a smooth switching-over can be achieved. In the conventional four-wheel drive automobile, the front transmission section is provided on the side of the engine, and adjacent to the transmission section is provided a transfer for the four-wheel drive which comprises two output shafts, clutch and links. Furthermore, behind the transfer is provided the rear transmission section to which lubricating oil or hydraulic operating fluid is to be applied and introduced. The transfer is isolated by a partition from the front and rear transmission sections. The transfer case in which the transfer is housed is radially larger, and its bottom is usually located lower than the that of a casing which houses the front and rear transmission sections. An oil tank is provided under the bottom of the front transmission section, and oil from the tank is fed by an oil pump to the two sections of the transmission. When the lubricating oil or hydraulic operating oil drained from the rear transmission section, among others, is returned to the oil tank, it is difficult for the oil to flow back to the tank smoothly or the oil tends to stay in the transfer case because the above-mentioned partition in the transfer prevents the flow of the oil.